I'm not an expert, but I've fooled around with a few radios...
I'm thinking that your 1965 radio won't have any RCA pre-amp outputs on the head unit. That means that if you want an external amp, you'll need one with speaker level inputs to accept your radio's speaker wires. I think most amps come with these, but it would be good to check before purchasing your new amp.
I'm thinking that a mono radio will have only 2 speaker wires (+ & -), those are what you hook to the new amp. I know some of the old radios ran the - speaker wire to ground, so you may only have the + wire on your radio. If that's the case, I think you can get away with continuing to run the - wire from ground, but todays manufacturers don't reccomend it because the vehicle ground can cause unwanted noise. I'm thinking that a mono radio will have only 1 channel with no rear fader, so you would need only a 1 channel amp. I'm not sure what year the optional external rear speaker fader was available from the factory, but if you hook up one of those, then you MAY be able to split your one channel into two channels, which you could use for either front & rear as intended, or right & left. In that case, you MAY be able to use a 2 channel amp.
Now for the complex part... Speakers can be wired in series or in parallel. When wiring speakers in series, you simply add the ohms of each speaker together to arrive at your total ohms. More ohms = more resistance to the powerflow, so your amp will run cool, but you will need a bigger watt amp to provide the same volume that a smaller amp could do if it had less resistance. Also, Series wiring isn't usually done because it makes the speakers fire out of phase with one another, which can sound a little odd. Parallel wiring is what is normaly used for speakers because it keeps them firing in phase with eachother for a clean sound. The problem with parallel wiring is that it reduces the total ohms. Here's the ohm formula for parallel wiring... Divide 1 by the ohm rating for your first speaker. Do this same thing for each of your speakers. Add all the answers together. Divide 1 by the total of your answers. Amps will be rated as being stable at a certain ohm level. 4 ohm stable & 2 ohm stable are fairly common. I've also heard of 1 ohm stable, but I suspect that would be very pricey. Less ohms = less resistance to the powerflow, so you'll have louder speakers, but if your ohm level drops below your amps ohm stable rating, then you can draw more power than your amp can handle which will cause it to overheat and meltdown inside.
As an example, two 4 ohm speakers wired in series = 8 ohms. two 4 ohm speakers wired in parallel = 2 ohms. five 4 ohm speakers wired in parallel = .8 ohms, which will melt just about any amp. Even if you were to use home entertainment speakers (which are usually 8 ohm), five 8 ohm speakers wired in parallel = 1.6 ohms.
I like to measure each of my speakers ohms with a digital multimeter. I find that sometimes the actual speaker doesn't quite match the ratings on the package. I've seen several 4 ohm speakers that were actually 3.1 ohms. Another thing to keep in mind is that the length and quality of your speaker wire can add some resistance. I like to measure my speakers ohms with the speaker wire I plan to use attached, so that I can see what kind of ohm load the amp will actually be dealing with.
You can combine series and parallel wiring to reach an acceptable ohm load, but you'll still have the out of phase problem with the series wired speakers. Imagine listening to the same song on two CD players, but one of the songs was started a fraction of a second after the other.
My reccomendation... The 1965 radio does not belong in a 1970 charger anyway. Upgrade to a modern head unit with 4 channels (front & rear, left & right) If you still want an external amp to go with it, choose a head unit with RCA pre-amp outputs.
If you'd rather have a vintage looking radio, talk with BigMoneyLewis. He's the vintage radio repair guy, and a member here on moparts.
As far as using 5 speakers, there's a few different ways you can go.
1. Most people would skip the center dash speaker and just use the four corner speakers.
2. Another option is to hook the center speaker parallel to the front right, but you'll need to make sure you're 2 ohm stable.
3. You could also fabricate a plate to mount two small speakers in the center dash speaker hole and parallel wire one to the left front and the other to the right front, but you'll still need to make sure you're 2 ohm stable.
4. Buy an expensive sound processor for hooking true surround sound up in in your car. This is normally used for hooking up DVD players in automotives, and will give you a true center channel to hook your center dash speaker to.
5. Go all stock with an original crossover box and original 5 speaker setup.
Don't try to use the original crossover box with a new head unit. Your center dash speaker won't be amplified, and you won't be able to hear it anyway.
Tav
